Overpeck gave the Bjerknes Lecture at AGU, modestly entitled “Anticipating the Big Impacts of Anthropogenic Carbon Dioxide Increases”. It was in the largest forum — about 1000 people were there. The room was filled to overflowing and you simply could not get in if you were late.

From his website, Overpeck seems to be about 50, but he is youthful looking. A full crop of hair undoubtedly helps here. He had a young child (toddler) with him at the conference. He has been proposed as the most likely person to have uttered the phrase “We have to get rid of the MWP” and Overpeck et al [1997] was one of the early entries in the multiproxy endeavour.

At the U.S. Climate Change Science Program meeting which I attended a few weeks ago, some of the sessions complained that people didn’t really care very much about a degree or two of temperature. However, they did care about drought and sea level. So there seemed to be a growing view that achieving climate policy objectives in the U.S. meant connecting with people on these issues that they cared about.

It’s hard not to conclude that Overpeck was attuned to the market and giving them what they wanted. You want sea level — Overpeck had sea level. You want drought — Overpeck had that too. Bob Carter described the session as being like being at an evangelist’s meeting. It hardly seems coincidental that the hot-button issues should have featured so dramatically.

I missed the first part of his talk on sea level but heard it mentioned in the summary, where Overpeck talked about a 6 meter possible rise. I did hear the section on drought.

Curiously, his talk was spent mostly on discussions of past droughts as determined by tree ring records. Overpeck referred to studies by Cook et al (which I’ve discussed briefly here) and by Hirschboek showing that past droughts in the U.S. West were far greater than anything experienced in recent history — none of this “wimpy” [his word] 20th and 21st century stuff. In the 1200s, there was a 50 year drought. He pointed out that the 20th century was a “pluvial” and that water allocation in the 1920s was in one of the wettest decades in the tree ring records. He mentioned the following link

The hook to global warming was that global warming would make such droughts even worse — a “super interglacial drought” or even a “global change-type drought”.

I guess that even evangelists sometimes have their moments of reflectiveness. Overpeck concluded the speech to his adoring audience by saying somewhat disarmingly: “You didn’t really believe everything that I said, did you?” I thought that it was a good question, but no one else seemed to notice.

32 Comments

Nice. After my time in the trenches of science, I think the line from the Pink Floyd song Have a Cigar sums it up – “And did we tell you the name of the game, boy, we call it Riding the Gravy Train” – I am afraid it’s a basic career skill.

A 6 meter rise in sea level, or drop in topography – same thing if one thinks about it. Iam still wondering how the ancient city of Alexandria, on the Nile delta, which is 20 metres below sea level, got there.

Re #4: I don’t get it either. The current mantra is: "wet gets wetter, dry gets drier". This is certainly not obvious and indeed it’s counter-intuitive. When you think about it, this is really a nut-cutting issue and it’s well worth examining closely any bases for making this claim as well as noting carefully exactly who’s making the claims. I specifically heard "wet gets wetter.." at the CCSP Workshop, but I don’t recall who it was attributed to.

The 6 meters referred to the 4-6 meters higher sea level demonstrated during the last interglacial (Eemian) under conditions that were ~1-2 C warmer than today on average. The current community view is that one could expect ~1 m to come by 2100 and up to 6 after several hundred years. An argument has been made that such melting (4-6 meters worth) will already be committed (i.e. unstoppable) by 2100 if no mitigating action is taken this century. He also argued based on evidence from the Larsen Ice Shelf collapse and paleoreconstructions of the last interglacial that the climate system might be able to deliver substantially more than 1 m sea level rise by 2100.

The argument for a drier Western US comes from tree rings and other proxies that show longer, more severe droughts in the region during the Medieval Warm Period. So, if you assume that MWP warming is real and the proxies are right, then warmer implies less precipitation, at least in the specific case of the Western US.

Which begs the questions: “If the current warming is caused by greenhouse gases, and those gases are present in concentratrions unprecedented in the last 650,000 years, why isn’t the temperature unprecedented?” and “Why are the temperatures during the Holocene generally less that the Eemian?” and “Why was the Eemian warmer?”

I’d be curious to know what other indicators show the Western US to be dry during the MWP rather than retarding the growth of trees because it was too warm?

(you might need a subscription to read this online: the reference is Science 25 February 2005: Vol. 307. no. 5713, pp. 1192 – 1194 if you care to employ the antiquated library methodology of information acquisition)

Here are the paragraphs most relevant to your question:

“Meanwhile, geologist Stanley has examined dozens of cores from the harbor and uncovered evidence of the centuries-long battle that ancient engineers waged against both gradual and sudden subsidence. He says the subsidence was brought on by a lethal combination of earthquakes, tsunamis, and the slow but relentless sinking of heavy foundations into unstable soil, which defeated even savvy Roman engineers. Although several wharves appear to have been reconstructed over centuries, no amount of piling could long hold up heavy stone foundations and buildings, he says. “[Adding] on all that material was asking for trouble,” Stanley says. “The additional weight of a wave surge could be powerful enough” to submerge part of Alexandria’s shore.”

“The historical record also shows an unusually active period of tremors from the 4th to the 6th centuries C.E. Quakes and tsunamis could have transformed sediment into a more fluid state, says Stanley. Sixty-five cores taken from the western harbor show signs of ancient liquefaction, he said, and numerous pieces of red coral not native to the harbor suggest that a tsunami washed them into the basin. But he says it is too early to reconstruct details of ancient collapse and rebuilding. “We need better 3D images of harbor substrate” to understand what repairs were done and when, he said.”

“The impact of these geological forces extended beyond Alexandria–and with even more dramatic consequences. Stanley and Goddio also are excavating three submerged cities in nearby Aboukir Bay:Herakleion, Canopus, and Menouthis. The first was an important entrance point to the mouth of the Nile, and the others were well-known pilgrimage sites. The area received huge amounts of sediment from the Nile, which compacted and sank over time. This process, combined with a slow rise in world sea levels, pushed the water at least 2 meters higher between the 6th century B.C.E. and the 7th century C.E. “Arabic texts show a huge Nile flood in 741 and 742 A.D.,” notes Clauss. And by the 8th century–the same time Alexandria slips into obscurity–the historical record on these sites goes silent.”

“Stanley’s research supports a theory that combines catastrophe and gradual sinking to explain the disappearance. Submergence alone cannot explain why much of the area is now a full 6 meters under water, and Stanley posits that sudden shifts in the flow of Nile branches on the delta–perhaps brought on by the spate of earthquakes–may have triggered more dramatic changes. Unstable sediment would have been laterally displaced, causing sudden destruction as the Nile moved into a new bed. Goddio’s team has found evidence of human remains underneath toppled walls at the three sunken cities, backing up this theory.”

Re #11: DF, while Overpeck may seem a little opportunistic, I’ll acknowledge that it’s fair enough to point out other implications of warming if people are insufficiently motivated by the prospect of their weather being more similar to the present temperature of cities 100 miles to the south.

However, the attribution of drought in the U.S. West to warming, as a specific example, if based on the precedent of drought in the US West in the MWP, does somewhat seem like sucking and blowing, if you also deny the existence of the MWP as a warm period outside the North Atlantic, as many of these authors do.

I can also see a distinction between the blanket phrase “wet gets wetter and dry gets drier” as applied to the world as a whole and a specific estimate that the U.S. West gets drier. Perhaps this is a precedent in the US West, but my recollection is that parts of Africa became moister in the MWP. If the “wet gets wetter, dry get drier” phrase is not true, then one is involved in a regional calculus. Is the outcome necessarily bad? I don’t know. But I’m suspicious of slogans that are too glib.

“The 6 meters referred to the 4-6 meters higher sea level demonstrated during the last interglacial (Eemian) under conditions that were ~1-2 C warmer than today on average.”

That may have been the total rise, but how long did it take to get 4-6 Meters higher? Current rates of sea rise attributed to warming is approx 0.42 mm a year or .016 inches per year.

“The current community view is that one could expect ~1 m to come by 2100 and up to 6 after several hundred years. An argument has been made that such melting (4-6 meters worth) will already be committed (i.e. unstoppable) by 2100 if no mitigating action is taken this century.”

Can we examine that for a moment. First let us look at the One Meter. We would have to see sea rise of approx a 1/3 of an inch every year for the next 100 years. Anything less now has to be made up for later. Granting that this is total sea rise, and not just warming. That is approx 3X to 5X more than the total sea rise we are currently seeing. So we are a bit behind the eightball. And that is total, it is approx 39X (that is 3900%) more than we are seeing due to warming alone currently.

And that’s for 1 Meter.

For 4 and 6 take the above numbers and multiply them by 4 and 6 of course.

For 4 meters rise we need 1.56 inches of rise per year or 12X to 20X more than we are currntly seeing for all reasons. 156X more than we are currently seeing due to warming.

For 6 meters rise we need 2.3 inches of rise per year or 18X to 30X more than we are currntly seeing for all reasons. 234X what we are currently seeing due to warming.

As an aside. The issue is Melting of landbound Ice in Greenland, as melting ice in the north polar region will have no effect on sea level, and currently the south polar region seems to be hoarding more water than it is releasing. So with Greenland Ice fueling this, supposedly it will shut off the North Atlantic current, which will make that area in the Norhtern Hemisphere colder, which means more Ice, not less, in Greenland, which means sea level rise slows down. No?

Yes there are other Glaciers worldwide which could be attributing, but a simple glance at a map or a globe shows us that they are many times less in size, in total, than Greenland.

If the Eemian is to be used as the analog, then you get to 4-6 meters by taking roughly equal amounts out of Greenland and West Antarctica and about ~0.5 m out of mountain glaciers. Whether the Eemian is a good analog is of course open to interpretation since the forcing pattern associated with somewhat stronger Milankovitch effects is noticably different than that predicted for CO2 rise, especially in the neighborhood of Antarctica, where greenhouse gas forcing is anticipated to be significantly stronger.

Let me clarify something I said earlier. ~1 m of sea level rise is the most that most members of the climate community believe is realistically possible and presumes acceleration. (Overpeck argued that we could exceed that.) A more central value, as represented by the IPCC, is ~0.5 m by 2100.

With respect to Antarctica’s contribution to present sea level change, I would say this is still an open question. Some instruments (e.g. ICESat) point to a net accumulation in Antarctica, others (e.g. GRACE) point to a net loss from Antarctica. Personally, I’d be more inclined to believe the GRACE data as that appears to be an easier measurement to make, but the community obviously needs to resolve this discrepancy.

…except for the inconvenient fact that the Greenland Ice Sheet didn’t completely melt in the Eemian period either. It’s been around for about 2 million years.

I’ve no idea whether this extra fresh water for the collapse of the THC will come from. But I’m certain that however implausuble the reason, it will be entirely ascribed to “a discernable human influence”.

I said earlier. ~1 m of sea level rise is the most that most members of the climate community believe is realistically possible and presumes acceleration

Whose is this “most members of the climate community” that you speak of? Did you take a poll? Was there a vigorous debate? Can we see the data used and the methodology?

You may not have realised, but the Dutch have already examined the issue of sea level rise, which I mentioned back in the mists of time. They have most to lose from “accelerating sea level rise” – at least 60% of the country is below sea level.

Your still talking about total possible, but not about time, you still haven’t shown where we will see an exponential increase in sea level increase. We can argue about how much will come from where, you still don’t show how a rate currently at 0.42 mm a year translates into 1 meter in 100 years, much less 4 – 6 meters.

We can all agree that as the Earth warms ice will melt. It’s a phenomenon most of us see everyday. What I’m arguing is the AMOUNT of melt. At current rates to rise 1 Meter would take more than 2000 years even halving that assuming DRASTIC increase would be 1000 years. It’s not that the numbers are off, it’s that they aren’t even in the ballpark.

Funnily enough 1 meter is the least amount I’ve heard bandied about for 100 years, I’ve seen claims as much as 15 meters (London being Underwater in 100 years)

Maybe you prefer a different source, but the IPCC makes it 1-2 mm/yr from 1900-2000 and the TOPEX/Poseidon satellites are putting it at ~3 mm/yr over the last decade (which is still consistent with 2 mm/yr, given the uncertainty and natural variability). A value of 2-3 mm/yr gets 20-30 cm by 2100, so it wouldn’t be a big stretch to get to 50 cm with a little bit of acceleration.

In terms of deglaciations there is good evidence of episodes of rates of 1 m / century and so-so evidence (e.g. meltwater pulse 1A) of bursts to rates of ~3-5 m / century. Whether such rates could be achieved in present times with much smaller ice sheets depends on who you want to believe about the stability of the modern ice sheets (i.e. Overpeck argued that burst like rates are possible in modern times from West Antarctica).

You should at least add the ~0.5 mm/yr attributed to thermal expansion. But in terms of budgeting there is an acknowledged problem that observed sea level rise is roughly a factor of 2 greater than estimated sea level rise based on counting up reservoirs.

Admittedly there may be important non-glacial / non-thermal sources of recent sea level rise that won’t participate in future sea level rise, but without some good accounting of what they are, I am inclined to believe that most of the difference between the estimated pre-1900 sea level rise of 0.1-0.2 mm/yr and the recent 1-2 mm/yr is probably attributed to glacial/thermal sources.

There was a very interesting article on sea level rise in the March 2002 “Physics Today,” here: http://www.physicstoday.org/pt/vol-55/iss-3/p35.html. The authors say that the global sea level (GSL) rise today is tiny, especially compared to what occurred at the end of the last glacial period. After that, sea level rise was about zero for the last several thousand years, and began again suddenly in the middle of the 19th century. That excludes the initiation of GSL rise from any connection to anthro-CO2.

They continue that there has been no detected acceleration in GSL rise over the entire 20th century. That means whatever was causing GSL to rise in the mid-19th early-20th centuries slowed and stopped at the identical but inverse rate of a new GSL rise due anthro-CO2, making the net rate of GSL rise small and constant. That convenient happenstance sounds highly unlikely.

Finally, the authors go on to discuss secular changes in land surface levels due to the continuing rebound from the loss of the weight of the glaciers. Compression of the land beneath the glaciers produced a ‘forebulge’ upward beyond the glaciers. Where the ‘forebulge’ is near what is now the shore, its subsidence will look like a sea-level rise. This effect is apparently complicating attempts to determine GSL rise worldwide, and is worth about 1 mm/yr. That’s about half the total estimated sea level rise. They say if they’re right about this, then neither Greenland nor Antarctica are contributing anything to sea level rise, i.e., neither are anomalously melting. The net GSL rise of about 1 mm/yr. is entirely explained by the melting of small ice-sheets and glaciers and by the thermal expansion of the oceans.

“You should at least add the ~0.5 mm/yr attributed to thermal expansion”

I would if anyone had any good numbers, everything to date I have seen is an estimate and usually combines thermal expansion and glacial melt. It is a very difficult thing to estimate due mainly to the fact that the depths of the oceans vary dramatically, and thermal expansion is not consistent along the entire depth. Thermal expansion genrally only happens at the surface, deep water being highly insulated and realitvely stable temprature wise. that still does not account for a drastic increase.

“Given that, I’m more inclined to believe the tide guage record of 1-2 mm/yr than our ability to estimate how much glaciers have decreased with time. E.g.”

Apples and oranges again. Tidal Gauge is total change, not just GW or AGW contributions. The majority of sea level rise is still accounted for by natural means other than GW, as such should stay realitively contant in a “Warming world” The oceans have been filling for Billions of years and those processes are still in place and will continue to contribute, but they are not thermally dependent (to any reasonable degree). Unless you believe in some form of intelligent deisgn where on the sixth day God said let there be water, and there was and he saw it was good. On the seventh day he took a bath. In this discusion we are concerned with the GW effect on sea level. You are measuring total sea level rise. By definition the amount of sea level rise attributed to GW is always going to be less than what the tidal gauge reads. Willis Eschenbach has a nice article on that, and most importantly he points out that the tide gauge record is woefully incomplete, even worse so that the surface temprature record.

” I am inclined to believe that most of the difference between the estimated pre-1900 sea level rise of 0.1-0.2 mm/yr and the recent 1-2 mm/yr is probably attributed to glacial/thermal sources.”

I have absolutely no confidence in the ” 0.1-0.2 mm/yr” number, and certainly you’ve not made a case for A=B, particuarly if the .1-.2 # comes from a cooling world (Little Ice age) or even a full ice age. A signifigantly cooler world, where my home was covered with a kilometer of ice is going to signifigantly reduce sea level rise.

Awww C’mon Evan it’s Christmastime. I wasn’t going to ruin the warmers Holidays by bringing that up, and that with the net runoff from glaciers its more like 100,000 years before it melts.

It’s bad enough they had to freeze their arses off during their Montreal Global Warming conference. Least you could do is leave them the Greenland Ice sheet melting for a couple of months while it’s confirmed.

Well who knows. Maybe they won’t have a white Christmas. Maybe that will cheer them up.

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[…] Steve McIntyre, fresh from AGU, has noticed Jonathan Overpeck talking about drought and sea level rise, inferring some newly minted marketing message: At the U.S. Climate Change Science Program meeting which I attended a few weeks ago, some of the sessions complained that people didn’t really care very much about a degree or two of temperature. However, they did care about drought and sea level. So there seemed to be a growing view that achieving climate policy objectives in the U.S. meant connecting with people on these issues that they cared about. […]